Identification of Tetrodotoxin and Fish Species in an Adulterated Dried Mullet Roe Implicated in Food Poisoning

Development of standardized methodology for identifying toxins in clinical samples and fish species associated with tetrodotoxin-borne poisoning incidents

Tetrodotoxin (TTX) is a naturally occurring toxin in food, especially in puffer fish. TTX poisoning is observed frequently in South East Asian regions. In TTX-derived food poisoning outbreaks, the amount of TTX recovered from suspicious fish samples or leftovers, and residual levels from biological fluids of victims are typically trace. However, liquid chromatography–mass spectrometry and liquid chromatography–tandem mass spectrometry methods have been demonstrated to qualitatively and quantitatively determine TTX in clinical samples from victims. Identification and validation of the TTX-originating seafood species responsible for a food poisoning incident is needed. A polymerase chain reaction-based method on mitochondrial DNA analysis is useful for identification of fish species. This review aims to collect pertinent information available on TTX-borne food poisoning incidents with a special emphasis on the analytical methods employed for TTX detection in clinical laboratories as well as for the identification of TTX-bearing species.

Analytical challenges: determination of tetrodotoxin in human urine and plasma by LC-MS/MS

Tetrodotoxin (TTX) is a powerful sodium channel blocker found in puffer fish and some marine animals. Cases of TTX poisoning most often result from puffer fish ingestion. Diagnosis is mainly from patient’s signs and symptoms or the detection of TTX in the leftover food. If leftover food is unavailable, the determination of TTX in the patient’s urine and/or plasma is essential to confirm the diagnosis. Although various methods for the determination of TTX have been published, most of them are for food tissue samples. Dealing with human urine and blood samples is much more challenging. Unlike in food, the amount of toxin in the urine and blood of a patient is generally extremely low; therefore a very sensitive method is required to detect it. In this regard, mass spectrometry (MS) methods are the best choice. Since TTX is a very polar compound, there will be lack of retention on conventional reverse-phase columns; use of ion pair reagent or hydrophilic interaction liquid chromatography (HILIC) can help solve this problem. The problem of ion suppression is another challenge in analyzing polar compound in biological samples. This review will discuss different MS methods and their pros and cons.

Toxicity and Molecular Identification of Green Toadfish Lagocephalus lunaris Collected from Kyushu Coast, Japan

Green toadfish Lagocephalus lunaris inhabits tropical and subtropical seas and contains high tetrodotoxin (TTX) levels in the muscle as well as liver and gonad. In 2008 to 2009, food poisoning due to ingesting L. lunais occurred in Western Japan. Five specimens of green toadfish caught in Kyushu coast, Japan, were analyzed for toxicity, toxins, and species identification. All five specimens were toxic by bioassay. Comparing the maximum toxicity in tissues, ovary contained the most toxin (1810 mouse unit [MU]/g), followed by liver (341 MU/g), muscle (135 MU/g), skin (79 MU/g), and intestine (72 MU/g). Liquid chromatography/mass spectrometry analysis revealed that TTX was the major toxin. Nucleotide sequence analysis of the 16S rRNA gene fragment of muscle mitochondrial DNA indicated that partial sequences of PCR products of four specimens were identical with that of L. lunaris. The sequence of one specimen was indistinguishable from that of the brown-backed toadfish Lagocephalus wheeleri, a nontoxic species.

Liquid chromatography-tandem mass spectrometry determination of the toxicity and identification of fish species in a suspected tetrodotoxin fish poisoning

Suspected tetrodotoxin (TTX) poisoning was associated with eating unknown fish in April 2009 in Taiwan. After ingestion of the fish, symptoms of the victim included perioral paresthesia, nausea, vomiting, ataxia, weakness of all limbs, respiration failure, and death within several hours. The toxicity in the remaining fish was determined, with the mice exhibiting symptoms of neurotoxin poisoning. The implicated fish and deceased victim tissues were analyzed for TTX by liquid chromatography-tandem mass spectrometry. The urine, bile, cerebrospinal fluid (spinal cord), pleural effusion, and pericardial effusion of the victim contained TTX. In addition, the partial cytochrome b gene of the implicated fish was determined by PCR. The DNA sequence in the partial 465-bp cytochrome b gene identified the implicated fish as Chelonodon patoca (puffer fish). These results indicate that people should avoid eating unknown fish species from fish markets where harvested fish may include toxic species.

Tetrodotoxin Poisoning

Tetrodotoxin (TTX) is one of the most potent and oldest known neurotoxins. The poisoning cases due to ingestion of TTX-containing marine animals, especially for puffer, have frequently occurred in Asia since a long time ago. This chapter describes various topics on TTX poisoning including the tendency of poisoning incidents, typical case report, treatment and prevention, biology distribution, original source, infestation mechanism, detection methods, characteristics of chemistry and pharmacology, and therapeutic application. Furthermore, the protocols for how to make puffer safe to eat and how to prevent puffer products made from toxic puffers have been suggested. Finally, the biological significance and neurophysiological role of TTX have been elucidated and TTX may act as an important drug like anesthetic in future.

Molecular identification of pufferfish species using PCR amplification and restriction analysis of a segment of the 16S rRNA gene

This study amplified the mitochondrial 16S rRNA gene using polymerase chain reaction (PCR) with a template of total DNA from muscle tissues of nine pufferfish species collected from the coastal area of Okinawa Islands in Japan: Pleuranacanthus sceleratus, Triodon macropterus, Chelonodon patoca, Sphoeroides pachygaster, Arothron hispidus, A. stellatus, A. manilensis, A. mappa, and A. nigropunctatus. Then nucleotide sequence encoding a partial region of the 16S rRNA gene was compared among species. The sequenced fragment was also used to select restriction enzymes, yielding species-specific restriction fragment length polymorphisms (RFLP). The sequence of the segment of the 16S rRNA gene consisted of about 615 nucleotides and showed interspecies variations in the targeted region. After calculation of corresponding RFLP-patterns of nine species investigated with suitable restriction enzymes, three restriction enzymes - BanII, DdeI, and NlaIII - were found to be sufficient for identification of all nine species. Successful testing of this methodology in frozen and heated food samples suggests its utility for pufferfish species authentication in food products.

Applying DNA techniques to the identification of the species of dressed toasted eel products

To differentiate the species of processed eel products, the gene identification of four fresh eel species was first established and the species of eel products collected from markets were investigated. Polymerase Chain Reaction (PCR) and sequence analysis were used to determine the genetic variation in a 362-nucleotide region of the mitochondrial cytochrome b gene in four fresh eels including Anguilla japonica, Anguilla anguilla, Anguilla rostrata, and Muraenesox cinereus. It was found that each eel species had a unique genotype, which was no different among fresh, frozen, and sterilized meats. The restriction enzyme HinfI could differentiate the species of A. japonica and A. rostrata but could not differentiate A. anguilla and M. cinereus. Another restriction enzyme, Sau96 I, was valuable in the differentiation of M. cinereus from the other three species of Anguilla. By applying PCR and restriction enzymes, the species of 12 commercial eel products were identified as A. japonica (9 samples), A. anguilla (2), and A. rostrata (1). This indicated that the sequence and restriction enzyme cutting site analyses were very usable to authenticate species of different processed eel products.

Molecular phylogenetic relationships of puffer fish inferred from partial sequences of cytochrome b gene and restriction fragment length polymorphism analysis

Phylogenetic relationships among puffer fish were investigated by comparing cytochrome b gene sequences and restriction endonuclease assays of 16 species from Taiwan. DNA was prepared for sequencing by PCR. No variation in sequences was detected among individuals within each species. Direct estimates of mitochondrial cytochrome b gene sequence divergence among 16 puffer fish were from 3.41 to 31.78%. Different restriction patterns were found among 16 puffer fish with 10 restriction endonucleases, whereas no variation in patterns was detected among individuals within each species. The polymorphisms obtained by RFLP have provided a new set of genetic markers for the accurate identification of sibling puffer species. It is the first molecularly based study of puffer diversity and sheds light on the evolution and taxonomy of this major puffer fish family.

Preliminary study on puffer fish proteome-species identification of puffer fish by two-dimensional electrophoresis

The aims of this work were to determine the differential characterization of the urea soluble protein components of puffer fish species and to establish a preliminary proteomic database using an immobilized pH gradient two-dimensional electrophoresis (2DE) technique. The puffer fish muscle proteins resolved into 171-260 spots in the 2DE gels, with a pI range of 3-10 and molecular mass range of 7.4-205.0 kDa, following Comassie blue staining. Puffer fish muscle proteins fell in the region with pI values of 3.5-7.0, and molecular masses of 7.4-45.0 kDa were well-resolved and were good for species comparison. The more acidic proteins of lower molecular masses showed species specific characteristics. Therefore, the species of puffer fish can be differentiated from the comparison of the characteristic 2DE protein patterns.